matrix-spec/content/rooms/fragments/v9-redactions.md

{{% added-in this=true %}} m.room.member events now keep join_authorised_via_users_server in addition to other keys in content when being redacted.

{{% boxes/rationale %}} Without the join_authorised_via_users_server property, redacted join events can become invalid when verifying the auth chain of a given event, thus creating a split-brain scenario where the user is able to speak from one server's perspective but most others will continually reject their events.

This can theoretically be worked around with a rejoin to the room, being careful not to use the faulty events as prev_events, though instead it is encouraged to use v9 rooms over v8 rooms to outright avoid the situation.

Issue #3373 has further information. {{% /boxes/rationale %}}

The full redaction algorithm follows.

Upon receipt of a redaction event, the server must strip off any keys not in the following list:

• event_id
• type
• room_id
• sender
• state_key
• content
• hashes
• signatures
• depth
• prev_events
• prev_state
• auth_events
• origin
• origin_server_ts
• membership

The content object must also be stripped of all keys, unless it is one of the following event types:

• m.room.member allows keys membership, join_authorised_via_users_server.
• m.room.create allows key creator.
• m.room.join_rules allows keys join_rule, allow.
• m.room.power_levels allows keys ban, events, events_default, kick, redact, state_default, users, users_default.
• m.room.history_visibility allows key history_visibility.